A number of hydrometallurgical processes have been developed to allow the separation and recovery of cobalt, nickel and several other metals from ammoniacal solutions, i.e. solutions of either ammonia/ammonium sulphate or ammonia/ammonium carbonate.
Briefly, the most common process is based upon the dissolution of cobalt and nickel as their ammine complexes, e.g. Ni(NH.sub.3).sub.6 2+, followed by selective preciptation. See, for example, Forward, F. A., and Mackiw, V. N., "Chemistry of the Ammonia Pressure Process for Leaching Ni, Cu, and Co from Sherritt Gordon Sulphide Concentrates", Trans. AIME, Vol. 203, pp. 457-463, 1955.
In general, precipitation involves several unit processes and results in a relatively impure product. Several solvent extractants have been investigated as an alternative method to precipitation with the object of improving on product purity. Both chelating (oximes) and acid extractants (di-(2-ethylhexyl)phosphoric acid, D2EHPA) have been studied.
However, neither group of reagents has proved ideally suited. It is extremely difficult to strip Co.sup.3+ from chelating extractants and reagents such as D2EHPA exhibit the same low Co--Ni selectivity in alkaline systems as they do in acid media.
Bis(2,4,4-trimethylpentyl)phosphinic acid (TMPPA) is a commercially available extractant for acid systems. TMPPA is well known to exhibit high Co--Ni selectively for example in acid sulfate and chloride systems. However, an acid system differs from an ammoniacal aqueous system in that the high pH would be expected to promote nickel extraction, and therefore high Co selectivity in an ammoniacal aqueous system would not be expected.
A process for the extraction of cobalt from ammoniacal solutions comprising cobalt and nickel whereby there is a high selectivity for cobalt and which utilizes a commercially available reagent as an extractant would be a most welcome contribution to the art. This invention provides just such a contribution.